Apparatus for detecting an amount of strain and method for manufacturing same

ABSTRACT

An apparatus for detecting an amount of strain comprises a strain generating part, an electrical insulating layer and sensing elements. The strain generating part is a member to which strain is to be applied. The electrical insulating layer is formed on the strain generating part. The sensing elements are formed on the electrical insulating layer. Each of the sensing elements is made of a silicon film. The silicon film comprises a poly-crystalline main layer and a poly-crystalline interface-layer, which comes into contact with the electrical insulating layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus for detecting anamount of strain, which is to be applied to measurement of pressure offluid such as gas or liquid, and to a method for manufacturing such anapparatus.

[0003] 2. Description of the Related Art

[0004] With respect to fabrication of sensing elements in theconventional strain detecting apparatus, there exists a technique toform poly-crystalline silicon films (strain gages) on a substrate sheetutilizing a plasma CVD method, as described in Japanese PatentPublication No. H6-70969.

[0005] In the conventional strain detecting apparatus fabricatedutilizing the above-mentioned method, an electrical insulating layer 31(i.e., a silicon oxide film) is formed on a diaphragm-main body 30,which has a cavity 30 a into which fluid to be subjected to a pressuremeasurement is to be introduced, and then, the poly-crystalline siliconfilms are formed on the electrical insulating layer 31 so as to serve asthe strain gages 32, as shown in FIG. 7.

[0006] More specifically, in such a strain detecting apparatus, thepoly-crystalline silicon film is formed at a temperature of up to 590°C. utilizing the plasma CVD method. Such a poly-crystalline silicon filmis subjected to a process such as photolithography to provide the straingages 32 having an appropriate shape.

[0007] Such strain gages 32 are advantageous in manufacturing thepoly-crystalline silicon films in large quantities and at low cost. Thestrain gage 32 has appropriate temperature characteristics for thestrain gage, i.e., TCR (temperature coefficient of resistance) of from−700 ppm/° C. to −200 ppm/° C. and TCS (temperature coefficient ofstrain) of ±300 ppm/° C., thus providing good characteristic propertiesfor the silicon film.

[0008] In the alternative method for forming poly-crystalline siliconfilms, an amorphous silicon film is formed at a temperature of up to590° C. utilizing a plasma CVD method or a sputtering method, theamorphous silicon film is subjected to a crystallization processutilizing a laser annealing method, and then the resultant silicon filmis subjected to a process such as photolithography to provide straingages having an appropriate shape.

[0009] It is specifically noted that the thus provided strain gage isformed of substantially poly-crystalline silicon film having arelatively large grain size. It is also possible to reduce an amount ofamorphous silicon left in the film to an excessively small amount.

[0010] However, the above-mentioned substantially poly-crystallinesilicon film, which serves as the strain gage 32 in the strain detectingapparatus, has a grain size of about 0.1 μm, and is provided on itslower side with an amorphous interface-layer 33 coming into contact withthe electrical insulating layer 31, as shown in FIG. 8. When acontinuous measurement of strain was made in a relatively hightemperature atmosphere (at least 100° C.) with the use of the straindetecting apparatus in which the above-mentioned substantiallypoly-crystalline silicon film is used as the strain gage, there observeda phenomenon (load characteristic at high temperature) in which the zeropoint for an output creeps as shown in FIG. 9. This occurs due to thefact that retarded elasticity of the amorphous interface-layer 33 causesthe strain gage itself to creep in a stress applying direction(compression or tensile direction).

[0011] In addition, the laser annealing process has to be applied toeach of sensing elements (i.e., the strain gages) to subject theabove-mentioned amorphous interface-layer 33 to the crystallizationprocess so as to provide the appropriate strain gages, thus beinginconsistent with mass production. Further, the grain size of thesilicon film exerts a significant influence on temperaturecharacteristic of the strain gage. The silicon film, which has beensubjected to the laser annealing process, has TCR (temperaturecoefficient of resistance) of about 2,000 ppm/° C. and TCS (temperaturecoefficient of strain) of −1,500 ppm/° C., thus being unsuitable for thestrain detecting apparatus.

SUMMARY OF THE INVENTION

[0012] An object of the present invention, which was made to solve theexemplified problems as described above, is therefore to provide anapparatus for detecting an amount of strain, which enables improvementin load characteristic at a high temperature without deteriorating thetemperature characteristic of a polycrystalline silicon layer formed onan electrical insulating layer, and a method for manufacturing such anapparatus.

[0013] In order to attain the aforementioned object, an apparatus of thefirst aspect of the present invention for detecting an amount of straincomprises:

[0014] a strain generating part to which strain is to be applied;

[0015] an electrical insulating layer formed on the strain generatingpart; and

[0016] sensing elements formed on the electrical insulating layer, eachof said sensing elements being made of a silicon film, said silicon filmcomprising a poly-crystalline main layer and a poly-crystallineinterface-layer, which comes into contact with the electrical insulatinglayer.

[0017] According to the features of the present invention, it ispossible to remarkably improve pressure load characteristic at a hightemperature, without deteriorating temperature characteristic of thesilicon film.

[0018] In the second aspect of the present invention, theabove-mentioned silicon film may be subjected, after formation thereof,to an annealing process at a temperature of from 540° C. to 590° C.

[0019] Limiting the upper limit of the temperature of the annealingprocess to 590° C. makes it possible to apply this treatment as aprecipitation hardening process as stated later, so as to improve thepressure load characteristic at a high temperature, withoutdeteriorating temperature characteristic of the silicon film. On theother hand, limiting the lower limit of the temperature of the annealingprocess to 540° C. makes it possible to crystallize the interface-layer,which comes into contact with the electrical insulating layer in theform of amorphous structure, so as to obtain the required specificresistance, thus remarkably improving the pressure load characteristicat a high temperature.

[0020] In the third aspect of the present invention, the above-mentionedstrain generating part may have a main body made of martensiticprecipitation hardened stainless steel, which comprises from 3 to 5 wt.% Ni, from 15 to 17.5 wt. % Cr and from 3 to 5 wt. % Cu.

[0021] According to such a feature concerning a suitable material forthe main body of the strain generating part, it is possible to impart ahigh elasticity and a high proof stress to the main body of the straingenerating part.

[0022] In the fourth aspect of the present invention, theabove-mentioned silicon film may contain an impurity as added in such amanner that specific resistance of the silicon film before saidannealing process is within a range of from 7×10⁻³ Ω·cm to 3.3×10⁻² Ω·cmand the specific resistance of the silicon film after said annealingprocess is within a range of from 3×10⁻³ Ω·cm to 1.7×10⁻² Ω·cm.

[0023] According to such a feature, it is possible to keep TCR(temperature coefficient of resistance) of the silicon film serving asthe strain gage within the range of from −300 ppm/° C. to +200 ppm/° C.,thus providing an excellent temperature characteristic. The loadcharacteristic at a high temperature of the strain detecting apparatuscan also be improved remarkably.

[0024] In the fifth aspect of the present invention, the above-mentionedimpurity may be boron.

[0025] In order to attain the aforementioned object, a method of thesixth aspect of the present invention for manufacturing an apparatus fordetecting an amount of strain, comprises the steps of:

[0026] (a) preparing a strain generating part to which strain is to beapplied,;

[0027] (b) forming an electrical insulating layer on said straingenerating part;

[0028] (c) preparing material for a silicon film; and

[0029] (d) forming the silicon film on said electrical insulating layer,utilizing said material to provide sensing elements thereon, saidsilicon film comprising a polycrystalline main layer and aninterface-layer, which comes into contact with the electrical insulatinglayer,

[0030] characterized in that:

[0031] said step (a) is carried out, utilizing martensitic precipitationhardened stainless steel, which comprises from 3 to 5 wt. % Ni, from 15to 17.5 wt. % Cr and from 3 to 5 wt. % Cu, to form a main body of saidstrain generating part;

[0032] said step (c) comprises adding boron as an impurity to saidmaterial for the silicon film so that specific resistance of the siliconfilm is within a range of from 7×10⁻³ Ω·cm to 3.3×10⁻² Ω·cm; and

[0033] said method further comprises (e) subjecting, after said step(d), said silicon film to an annealing process at a temperature of from540° C. to 590° C. so that the specific resistance of the silicon filmis within a range of from 3×10⁻³ Ω·cm to 1.7×10⁻² Ω·cm, thuscrystallizing said interface-layer.

[0034] According to the features of the present invention, it ispossible to manufacture the strain detecting apparatus, which permits toremarkably improve pressure load characteristic at a high temperature,without deteriorating temperature characteristic of the silicon film.

[0035] In the seventh aspect of the present invention, theabove-mentioned step (e) may be carried out in plasma into which gas isintroduced.

[0036] According to such a feature, it is possible to decrease thetemperature of the annealing process and reduce a required period oftime for the annealing process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1A is a cross-sectional view illustrating the straindetecting apparatus of he present invention, and FIG. 1B is a plan viewthereof;

[0038]FIG. 2 is an enlarged view of the portion surrounded with a circle“A” as shown in FIG. 1A;

[0039]FIG. 3 is a graph illustrating the relationship between TCRcharacteristic and an added amount of impurity;

[0040]FIG. 4 is a graph illustrating pressure load characteristic at ahigh temperature of a strain gage of the strain detecting apparatus, inconjunction with the relationship between the zero point outputvariation and a period of time;

[0041]FIG. 5 is a graph illustrating specific resistance characteristicof the silicon film before and after the annealing process;

[0042]FIG. 6 is a schematic structural view illustrating an example ofan annealing apparatus;

[0043]FIG. 7 is a schematic descriptive view illustrating theconventional strain detecting apparatus;

[0044]FIG. 8 is an enlarged view of the portion surrounded with a circle“B” as shown in FIG. 7; and

[0045]FIG. 9 is a graph illustrating pressure load characteristic at ahigh temperature of a strain gage of the conventional strain detectingapparatus, in conjunction with the relationship between the zero pointoutput variation and a period of time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Now, an embodiment of the strain detecting apparatus according tothe present invention will be described in detail below with referenceto FIGS. 1(a) to 5.

[0047]FIG. 1A is a cross-sectional view illustrating the straindetecting apparatus of he present invention and FIG. 1B is a plan viewthereof, and FIG. 2 is an enlarged view of the portion surrounded with acircle “A” as shown in FIG. 1A. As shown in these figures, the straindetecting apparatus 1 includes a metallic diaphragm-main body 5 servingas a strain generating part, an electrical insulating layer (hereinafterreferred to as the “insulating layer 10”) formed on the metallicdiaphragm-main body 5, strain gages 15, 15 formed on the surface of theinsulating layer 10 so as to serve as the sensing element, electrodepads 20, 20 formed on the insulating layer 10 so as to come into contactwith the strain gages 15, and a protecting film 25 formed on the straingages 15 and the electrode pads 20 so as to cover substantially theentirety of each of the strain gages 15, on the one hand, and theportions of the electrode pads 20, on the other hand.

[0048] The metallic diaphragm-main body 5 is composed of a tubularportion 8 and a thin film portion 7 for closing the upper opening of thetubular portion 8 to define a cavity 6 therein. The metallicdiaphragm-main body 5 is made of martensitic precipitation hardenedstainless steel, which comprises from 3 to 5 wt. % Ni, from 15 to 17.5wt. % Cr and from 3 to 5 wt. % Cu. The martensitic precipitationhardened stainless steel, which has an excellent elasticity and highproof stress, is a suitable material for the strain generating part. Themartensitic precipitation hardened stainless steel is usually obtainedby subjecting steel to a hot-rolling process, a solid solutiontreatment, and a precipitation hardening treatment, so as to provide amartensitic structure, which has a small amount of carbon, and in whichcopper-rich compounds are precipitated and hardened, thus imparting thehigh elasticity and the high proof stress to the steel.

[0049] Conditions for the solid solution treatment and the precipitationhardening treatment, which is to be carried out at the predeterminedprecipitation hardening temperature after completion of the solidsolution treatment, are shown in Table 1. TABLE 1 Kind of TreatmentSymbol Conditions Solid S Heating at 1020-1060° C. and then quenchingsolution treatment Precipitation H900 After Treatment “S”, heating at470-490° C. hardening and air cooling treatment H1025 After Treatment“S”, heating at 540-560° C. and air cooling H1075 After Treatment “S”,heating at 570-590° C. and air cooling H1150 After Treatment “S”,heating at 610-630° C. and air cooling

[0050] Mechanical properties of materials, which have been subjected tothe treatments according to the conditions as set forth in Table 1, areshown in Table 2. TABLE 2 Hardness Proof Tensile Elonga- Draw- HBSstress strength tion ability or HRC (N/mm²) (N/mm²) (%) (%) HBW (notKind of (not less (not less (not less (not less (not less less TreatmentSymbol than) than) than) than) than) than) Solid S — — — — 363 38solution treatment Precipitation H900 1175 1310 10 40 375 40 hardeningH1025 1000 1070 12 45 331 35 treatment H1075 860 1000 13 45 302 31 H1150725 930 16 50 277 28

[0051] It is recognized from the results as shown in Tables 1 and 2 thatthe temperature of the precipitation hardening treatment exertsinfluences on the mechanical properties of the material.

[0052] It is recognized from Tables 1 and 2 that application of theprecipitation hardening treatment “H900” in which the treatmenttemperature is kept within the range of from 470° C. to 490° C.,provides the maximum tensile strength of not less than 1310 N/mm². It isalso recognized that application of the precipitation hardeningtreatment “H1075” in which the treatment temperature is kept within therange of from 570° C. to 590° C., even provides the tensile strength ofnot less than 1000 N/mm².

[0053] The inner surface of the thin film portion 7 of thediaphragm-main body 5 serves as a pressure-receiving surface 6 a, whichcomes into contact with fluid such as gas or liquid having pressure tobe measured. The diaphragm-main body 5 is fluid-tightly connected at thelower end of the tubular portion 8 thereof to a conduit or container forthe fluid so that the fluid is introduced into the cavity 6 of thetubular portion 8.

[0054] The outer surface 7 a of the thin film portion 7 (i.e., the upper*surface of the diaphragm-main body 5), which extends to the outerperiphery of the tubular portion 8 on the opposite side to thepressure-receiving surface 6 a, is subjected to a mirror grinding.

[0055] The insulating layer 10 is formed on the above-mentioned outersurface 7 a. The insulating layer 10 is formed for example of a siliconoxide film. The insulating layer 10 is formed utilizing a plasma CVDmethod or a sputtering method.

[0056] The strain gages 15 are formed on the insulating layer 10 inpositions where compression stress and tensile stress are to be appliedto the diaphragm-main body 5. There are provided for example four straingages 15, i.e., two strain gages 15 provided in the positions to whichthe compression stress is to be applied, and the other two strain gages15 provided in the positions to which the tensile stress is to beapplied, as shown in FIG. 1B. In the present invention, the portion ofthe strain gage 15, which comes into contact with the insulating layer10, is called the “interface-layer”9.

[0057] The strain gage 15 is obtained by forming a crystalline siliconfilm containing boron as an impurity in thickness of about 0.5 μm,utilizing a plasma CVD method or a sputtering method, and thensubjecting the thus formed silicon film to the annealing process at atemperature of from 540° C. to 590° C.

[0058]FIG. 6 shows a schematic structure of an example of an annealingapparatus. The heating step as normally applied, which is included inthe above-described precipitation hardening treatment, is applied as theannealing treatment. Alternatively, the annealing apparatus, which isprovided with a heating system 40, a gas introducing system 41, a gasdischarging system 42 and electrodes 43, as shown in FIG. 6, may beutilized to carry out the annealing process along with the gasintroduction and utilization of plasma, while heating a substrate sheet44 to a temperature within the range of from 540° C. to 590° C. Then,the resultant crystalline silicon film is subjected to aphotolithography and a dry etching to form the strain gage 15.

[0059] The reason for limiting the upper limit of the annealingtreatment temperature to 590° C. is that application of the treatmentwith the thus limited temperature to the precipitation hardening processas shown in Tables 1 and 2 can maintain characteristics of the highelasticity and the high proof stress of the metallic diaphragm-main body5 (i.e., the main body of the strain generating part, which is made ofmartensitic precipitation hardened stainless steel, withoutdeteriorating the temperature characteristics of the strain detectingapparatus, and can improve remarkably the pressure load characteristicat a high temperature. The reason for limiting the lower limit of theannealing treatment temperature to 540° C. is that application of thetreatment at a temperature of less than 540° C. disables theinterface-layer 9 of the silicon film, which comes into contact with theinsulating layer 10, from being crystallized, thus leading to theexistence of an amorphous portion. As a result, the required specificresistance cannot be obtained, thus making impossible to improve thepressure load characteristic at a high temperature. The lower limit ofthe annealing treatment temperature should therefore be limited to atleast 540° C.

[0060] The crystalline silicon film includes boron as added as theimpurity. The boron is added in such a manner that the specificresistance of the silicon film before the annealing process is within arange of from 7×10⁻³ Ω·cm to 3.3×10⁻² Ω·cm. The specific resistance ofthe silicon film after the annealing process is within a range of from3×10⁻³ Ω·cm to 1.7×10⁻² Ω·cm.

[0061]FIG. 3 shows TCR (temperature coefficient of resistance)characteristic relative to an added amount of impurity, of the straingage serving as the sensing element of the detecting apparatus of thepresent invention. Addition of boron as the impurity enables the TCR ofthe strain gage to be kept within the range of from −700 ppm/° C. to−200 ppm/° C., as shown in FIG. 3. This makes it possible to attain therequirement that the TCR of the strain gage, which has been subjected tothe annealing process, is kept within the range of from −300 ppm/° C. to+200 ppm/° C. Thus, the strain gage having the excellent temperaturecharacteristic can be obtained.

[0062]FIG. 4 shows the pressure load characteristic at a hightemperature of the strain detecting apparatus of the present invention,to which thermal energy of 140° C. and pressure of 180 MPa are applied.Subjecting the silicon film, which has had the specific resistance offrom 7×10⁻³ Ω·cm to 3.3×10⁻² Ω·cm before the annealing process, to theannealing process at the temperature of from 540° C. to 590° C. enablesthe pressure load characteristic at a high temperature of the straindetecting apparatus to be decreased by up to 50 percent, in comparisonwith the apparatus, which has not been subjected to the annealingprocess, as shown in FIG. 4.

[0063]FIG. 5 shows the specific resistance characteristic of the siliconfilm before and after the annealing process. It is presumed thatapplication of the annealing process can improve crystallinity of thepoly-crystalline silicon film, thus occurring reduction of the specificresistance, as shown in FIG. 5.

[0064] The above-described annealing process enables the interface-layer9 of the silicon film (i.e., the strain gage 15), which comes intocontact with the insulating layer 10, to be poly-crystallized withoutexistence of an amorphous portion, as shown in FIG. 2. The crystallinesilicon film is the poly-crystalline body in which the crystals grow inthe form of columnar structure in the perpendicular direction to thesurface of the insulating layer 10.

[0065] The electrode pad 20 is provided so as to come into contact withthe insulating layer 10 and the strain gages 15, which are formed in thepositions where the compression stress and tensile stress are to beapplied. In addition, wiring for the strain gages 15 is carried out toform a full bridge circuit so as to output voltage in accordance withvariation of strain. The wires and the electrode pads 20 are formed ofmetallic thin film (such as an aluminum or gold film), utilizing avacuum evaporation or a sputtering method.

[0066] The protecting film 25 is provided to protect the contactportions of the strain gage 15 with the electrode pad 20.

[0067] The present invention is not limited only to the above-describedembodiment, and may be worked in the form of various kinds ofembodiments. The full bridge circuit having the four strain gages is notnecessarily used in the wiring for the strain detecting apparatusdescribed in the embodiment, and there may be used a half bridge circuitutilizing two strain gages or a Wheatstone bridge circuit utilizing thesingle strain gage.

[0068] The entire disclosure of Japanese Patent Application No.2002-287881 filed on Sep. 30, 2002 including the specification, claims,drawings and summary is incorporated herein by reference in itsentirety.

What is claimed is:
 1. An apparatus for detecting an amount of straincomprising: a strain generating part to which strain is to be applied;an electrical insulating layer formed on the strain generating part; andsensing elements formed on the electrical insulating layer, each of saidsensing elements being made of a silicon film, said silicon filmcomprising a poly-crystalline main layer and a poly-crystallineinterface-layer, which comes into contact with the electrical insulatinglayer.
 2. The apparatus as claimed in claim 1, wherein: said siliconfilm is subjected, after formation thereof, to an annealing process at atemperature of from 540° C. to 590° C.
 3. The apparatus as claimed inclaim 1, wherein: said strain generating part has a main body made ofmartensitic precipitation hardened stainless steel, which comprises from3 to 5 wt. % Ni, from 15 to 17.5 wt. % Cr and from 3 to 5 wt. % Cu. 4.The apparatus as claimed in claim 2, wherein: said silicon film containsan impurity as added in such a manner that specific resistance of thesilicon film before said annealing process is within a range of from7×10⁻³ Ω·cm to 3.3×10⁻² Ω·cm and the specific resistance of the siliconfilm after said annealing process is within a range of from 3×10⁻³ Ω·cmto 1.7×10⁻² Ω·cm.
 5. The apparatus as claimed in claim 4, wherein: saidimpurity is boron.
 6. A method for manufacturing an apparatus fordetecting an amount of strain, comprising the steps of: (a) preparing astrain generating part to which strain is to be applied,; (b) forming anelectrical insulating layer on said strain generating part; (c)preparing material for a silicon film; and (d) forming the silicon filmon said electrical insulating layer, utilizing said material to providesensing elements thereon, said silicon film comprising a polycrystallinemain layer and an interface-layer, which comes into contact with theelectrical insulating layer, characterized in that: said step (a) iscarried out, utilizing martensitic precipitation hardened stainlesssteel, which comprises from 3 to 5 wt. % Ni, from 15 to 17.5 wt. % Crand from 3 to 5 wt. % Cu, to form a main body of said strain generatingpart; said step (c) comprises adding boron as an impurity to saidmaterial for the silicon film so that specific resistance of the siliconfilm is within a range of from 7×10⁻³ Ω·cm to 3.3×10⁻² Ω·cm; and saidmethod further comprises (e) subjecting, after said step (d), saidsilicon film to an annealing process at a temperature of from 540° C. to590° C. so that the specific resistance of the silicon film is within arange of from 3×10⁻³ Ω·cm to 1.7×10⁻² Ω·cm, thus crystallizing saidinterface-layer.
 7. The method as claimed in claim 6, wherein: said step(e) is carried out in plasma into gas.